text: Mesohabitat types are defined by their hydromorphological units (HMUs), such as pools and rapids, geomorphology, land cover and other hydrological characteristics. Mesohabitats are mapped under multiple flow conditions at extensive sites along the river. Fish data is collected in randomly distributed mesohabitats where habitat surveys are also conducted. This allows modeling of available fish habitat at a range of flows. Rating curves represent the changes in relative area of suitable habitat in response to flow and allow for the determination of habitat quantity at any given flow within the range of surveys. These rating curves can be developed for river units of any size allowing conclusions to be drawn about the suitability of channel patterns or habitat structures for various species of fish for specific sections as well as for the entire river. Rating curves can also be used to evaluate the benefits of various restoration measures on the entire fish community. In combination with hydrologic time series, rating curves are used to create Continuous Under Threshold (CUT) curves for the analysis of frequency, magnitude and duration of significant habitat events. The CUT curve technique described by Capra et al. (1995)  defines critical thresholds and determines what habitat variability and availability is necessary to support the target river fauna. CUT curves evaluate durations of unsuitable habitat under a specified threshold by comparing continuous durations in days under this threshold to the cumulative durations in the study period. A highly useful product of the CUT curves are reference tables that can be used to determine how long a given species can tolerate unsuitable conditions depending on its life stage.